Various modes such as prepregs, yarns and glass mats (GMT) are known as molding materials composed of continuous reinforcing fiber bundles and thermoplastic resins. These molding materials containing thermoplastic resins have such features that molding work and storage are easy and that the molded articles thereof have high toughness and excellent recyclability. Above all, processed molding materials provided as pellets are useful as industrial materials since they can be applied to molding methods excellent in economic efficiency and productivity such as injection molding and stamping.
However, because of problems of economic efficiency and productivity in the molding material production process, they are not so widely used. For example, a thermoplastic resin excellent in toughness and elongation generally has a high viscosity and it is difficult to make a reinforcing fiber bundle impregnated with the thermoplastic resin. Therefore, such a thermoplastic resin is not suitable for easily producing a molding material at high productivity. On the other hand, if a thermoplastic resin with a low viscosity is used in view of easy impregnation, there arises the problem that the obtained molded article greatly declines in mechanical properties.
On the contrary, disclosed is a molding material in which a thermoplastic resin with a high molecular weight is disposed in contact with a composite material consisting of a thermoplastic polymer with a low molecular weight and continuous reinforcing fibers (for example, Patent Document 1).
This molding material can achieve both good economic efficiency and productivity and good mechanical properties by using a resin with a low molecular weight for impregnation into a continuous reinforcing fiber bundle and using a resin with a high molecular weight as a matrix resin. Further, if this molding material is molded by an injection molding method, the reinforcing fibers can be easily mixed with the matrix resin with the break loss of the fibers kept minimized in the stage when the material is plasticized at the time of molding, and a molded article excellent in the dispersion of fibers can be produced. Therefore, since the reinforcing fibers can remain longer than before in the obtained molded article, the molded article can have both good mechanical properties and excellent appearance quality.
However, in recent years, fiber-reinforced composite materials attract more attention and find expanded applications, and in this connection, a molding material more excellent in moldability and handling properties and capable of being molded into articles with more excellent mechanical properties is required. Further, higher economic efficiency and productivity are industrially required. For example, since fiber-reinforced composite materials are requested to be lighter in weight and higher in economic efficiency, light-weight olefin-based resins, particularly propylene-based resins begin to be used as matrix resins. Patent Document 2 discloses long-fiber pellets, in which carbon fibers treated by a sizing agent having a functional group capable of reacting with an acid radical are impregnated with an acid-modified olefin-based resin. However, the thermoplastic resin used in the pellets is an acid-modified olefin-based resin only, and since it has a high viscosity, it is very difficult to disperse a carbon fiber bundle. Consequently the molded article obtained from the long-fiber pellets has problems in mechanical properties and appearance.
Further, attempts are made to enhance the interfacial adhesiveness by treating the surfaces of reinforcing fibers or applying a sizing agent to the fibers, in order to enhance the mechanical properties of the molded article. Patent Document 3 describes a fiber treating agent containing a polypropylene-based resin modified by an unsaturated dicarboxylic acid or a salt thereof, and Patent Document 4 discloses applying an acid-modified polypropylene resin with an acid value of 23 to 120 mg KOH/g as a sizing agent suitable for polypropylene resin. However, neither of the proposals allows sufficient interfacial adhesiveness to be obtained.
Furthermore, Patent Document 5 discloses carbon fibers to which 0.1 to 8 wt % of an ionomer resin is adhered. Likewise, Patent Document 6 describes carbon fibers to which 0.1 to 8 wt % of two different acid-modified polypropylene-based resins are adhered. Both the patent documents are intended to enhance the interfacial adhesiveness between carbon fibers and a matrix resin by applying a polymer having affinity with an olefin resin to the carbon fibers.
However, as clearly described in these patent documents, since only a modified olefin component such as an ionomer resin or self-emulsifiable polypropylene-based resin is mainly adhered to reinforcing fibers, the film of the resin component is not sufficiently formed, and the interfacial adhesiveness and the handling properties of the fiber bundle cannot be sufficiently enhanced.    Patent Document 1: JP 10-138379 A    Patent Document 2: JP 2005-125581 A    Patent Document 3: JP 6-107442 A    Patent Document 4: JP 2005-48343 A    Patent Document 5: JP 2006-124852 A    Patent Document 6: WO 2006/101269